diff --git a/src/mesa/state_tracker/st_cb_syncobj.c b/src/mesa/state_tracker/st_cb_syncobj.c
index 123925a..265bf10 100644
--- a/src/mesa/state_tracker/st_cb_syncobj.c
+++ b/src/mesa/state_tracker/st_cb_syncobj.c
@@ -59,9 +59,13 @@ static void st_delete_sync_object(struct gl_context *ctx,
    struct pipe_screen *screen = st_context(ctx)->pipe->screen;
    struct st_sync_object *so = (struct st_sync_object*)obj;
 
+   mtx_lock(&ctx->Shared->Mutex);
+
    screen->fence_reference(screen, &so->fence, NULL);
    free(so->b.Label);
    free(so);
+
+   mtx_unlock(&ctx->Shared->Mutex);
 }
 
 static void st_fence_sync(struct gl_context *ctx, struct gl_sync_object *obj,
@@ -71,9 +75,14 @@ static void st_fence_sync(struct gl_context *ctx, struct gl_sync_object *obj,
    struct st_sync_object *so = (struct st_sync_object*)obj;
 
    assert(condition == GL_SYNC_GPU_COMMANDS_COMPLETE && flags == 0);
+
+   mtx_lock(&ctx->Shared->Mutex);
+
    assert(so->fence == NULL);
 
    pipe->flush(pipe, &so->fence, PIPE_FLUSH_DEFERRED);
+
+   mtx_unlock(&ctx->Shared->Mutex);
 }
 
 static void st_check_sync(struct gl_context *ctx, struct gl_sync_object *obj)
@@ -81,17 +90,32 @@ static void st_check_sync(struct gl_context *ctx, struct gl_sync_object *obj)
    struct pipe_context *pipe = st_context(ctx)->pipe;
    struct pipe_screen *screen = pipe->screen;
    struct st_sync_object *so = (struct st_sync_object*)obj;
+   struct pipe_fence_handle *fence = NULL;
+   boolean success;
+
+   mtx_lock(&ctx->Shared->Mutex);
+
+   screen->fence_reference(screen, &fence, so->fence);
 
    /* If the fence doesn't exist, assume it's signalled. */
-   if (!so->fence) {
+   if (!fence) {
       so->b.StatusFlag = GL_TRUE;
-      return;
+      goto unlock;
    }
 
-   if (screen->fence_finish(screen, pipe, so->fence, 0)) {
+   mtx_unlock(&ctx->Shared->Mutex);
+   success = screen->fence_finish(screen, pipe, fence, 0);
+   mtx_lock(&ctx->Shared->Mutex);
+
+   if (success) {
       screen->fence_reference(screen, &so->fence, NULL);
       so->b.StatusFlag = GL_TRUE;
    }
+
+   screen->fence_reference(screen, &fence, NULL);
+
+unlock:
+   mtx_unlock(&ctx->Shared->Mutex);
 }
 
 static void st_client_wait_sync(struct gl_context *ctx,
@@ -101,11 +125,17 @@ static void st_client_wait_sync(struct gl_context *ctx,
    struct pipe_context *pipe = st_context(ctx)->pipe;
    struct pipe_screen *screen = pipe->screen;
    struct st_sync_object *so = (struct st_sync_object*)obj;
+   struct pipe_fence_handle *fence = NULL;
+   boolean success;
+
+   mtx_lock(&ctx->Shared->Mutex);
+
+   screen->fence_reference(screen, &fence, so->fence);
 
    /* If the fence doesn't exist, assume it's signalled. */
-   if (!so->fence) {
+   if (!fence) {
       so->b.StatusFlag = GL_TRUE;
-      return;
+      goto unlock;
    }
 
    /* Section 4.1.2 of OpenGL 4.5 (Compatibility Profile) says:
@@ -120,11 +150,19 @@ static void st_client_wait_sync(struct gl_context *ctx,
     * Assume GL_SYNC_FLUSH_COMMANDS_BIT is always set, because applications
     * forget to set it.
     */
-   if (so->fence &&
-       screen->fence_finish(screen, pipe, so->fence, timeout)) {
+   mtx_unlock(&ctx->Shared->Mutex);
+   success = screen->fence_finish(screen, pipe, fence, timeout);
+   mtx_lock(&ctx->Shared->Mutex);
+
+   if (success) {
       screen->fence_reference(screen, &so->fence, NULL);
       so->b.StatusFlag = GL_TRUE;
    }
+
+   screen->fence_reference(screen, &fence, NULL);
+
+unlock:
+   mtx_unlock(&ctx->Shared->Mutex);
 }
 
 static void st_server_wait_sync(struct gl_context *ctx,